Oxydehydrogenation process for preparing methacrylic acid and its lower alkyl esters

ABSTRACT

Isobutyric acid or a lower alkyl ester thereof is oxidatively dehydrogenated in the vapor phase producing the corresponding α,β-olefinically unsaturated derivative by contact with a heterogeneous catalyst in the presence of molecular oxygen. The catalyst is composed of the calcined oxides of iron and at least two members selected from the group consisting of antimony, niobium, tantalum and tungsten.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the conversion of isobutyric acid or itsequivalents and lower alkyl esters thereof correspondingly tomethacrylic acid or its equivalents and lower alkyl esters thereof.

2. Description of the Prior Art

There exists considerable prior art relating to the oxydehydrogenationof the lower saturated aliphatic monocarboxylic acids to produce thecorresponding α,β-olefinically unsaturated acids. Early work in thisarea involved thermal, vapor phase oxydehydrogenation of the saturatedaliphatic carboxylic acid in the presence of oxygen and iodine. Thisapproach has not been particularly successful from a commericalstandpoint. This is understandably so inasmuch as iodine is costly andexhibits extreme corrosivity properties and recovery of comparativelylarge amounts thereof is required in the process. The heterogeneouscatalytic method for oxydehydrogenation according to the prior artappears to be the more attractive route to the commercial production ofolefinically unsaturated monocarboxylic acids. The prior artheterogeneous oxydehydrogenation catalysts useful for this purposeinclude some heteropoly acids such as phosphomolybdic acid optionallywith tungsten and/or vanadium. Another type of catalyst included in theprior art is iron phosphate. For instance, U.S. Pat. No. 3,948,959discloses that an alkali or alkaline earth metal can be included in aniron/phosphate catalyst useful for the oxydehydrogenation of isobutyricacid to methacrylic acid.

SUMMARY OF THE INVENTION

In accordance with this invention, a catalytic process is provided forthe oxidative dehydrogenation of a saturated aliphatic monocarboxylicacid or lower alkyl ester thereof, such as isobutyric acid or methylisobutyrate, to the corresponding α,β-olefinically unsaturatedderivative thereof, such as methyacrylic acid or methyl methacrylate.The process of this invention comprises contacting a heterogeneouscatalyst at a temperature in the range of from 300°-500° C. with amixture of the saturated aliphatic monocarboxylic acid and molecularoxygen, said catalyst being a calcined mixture of the oxides of iron andat least two members of the group consisting of antimony, niobium,tantalum and tungsten. The catalyst useful in this invention can befurther defined by the gram-atom empirical formula FeM_(a) O_(x) whereina is a number from 0.5 to 3 and M represents at least two membersselected from the group consisting of antimony, niobium, tantalum andtungsten and x represents a number determined by the valence requirementof the other elements present.

DESCRIPTION OF THE PREFERRED EMBODIMENT

There are a number of techniques which can be used for the preparationof the catalyst useful in the process of this invention. Of these, themore facile methods involve preparing the integral catalyst compositionprior to calcination. This can be readily and conveniently accomplishedby the so-called slurry method in which metal salts, either soluble ornot, are mixed in a liquid medium such as water, the water is removedand the resulting solid is calcined producing the desired catalyst. Ingeneral, any compounds containing the desired catalyst components may beused provided that they result, upon calcination in the oxides of theinstant catalyst. Suitable calcination temperatures range from400°-1000° C. Applicable periods of calcination range from 2-30 hoursalthough longer periods can be used without adverse results.

The use of a support or carrier for the catalyst is within the scope ofthe invention. The support can be included in the slurry preparationmentioned above. Useful carriers include alumina, pumice, siliconcarbide, zirconia, titania, silica, alumina-silica, etc. The supportwhen used can constitute from 3 percent to 99 percent and preferablybetween 5 percent and 95 percent by weight of the finished catalyst.

The process of this invention can be carried out using catalyst in theproper form for a fluidized bed reactor, a stirred tank reactor or for afixed or packed bed reactor or any combination of these types ofreactors. Because of the convenience associated with the use of a fixedbed reactor in a small scale operation, such a reactor will beexemplified herein. In the preferred mode of operation the feed to thereactor comprises a pre-heated gaseous mixture of the saturatedaliphatic monocarboxylic acids, molecular oxygen, steam and inertdiluent gas. In the case where methacrylic acid is produced fromisobutyric acid it may be desirable to include some acetone in the feedto the reactor. A preheat temperature in the range of about 300° to 350°C. is customarily used. The oxydehydrogenation reaction can be carriedout in the range of from 300° to 500° C. More generally a temperature offrom 375° to 475° C. provides for optimum processing.

The mole ratio of molecular oxygen to carboxylic acid is from 0.2 to 1.5and more preferably from 0.3 to 0.75 in the case where the carboxylicacid is isobutyric acid, per se. Although steam is not necessary for thereaction, its presence is desirable in the feed because it is believedto act beneficially as a heat sink and in minimizing combustion of thecarboxylic acid to undesirable products. The mole ratio of water to thecarboxylic acid in the feed should be from about 2 to 20. The optimumratio is from 6 to 12.

Another important parameter is the concentration of the organic reactantin the feed. The organic reactant carboxylic acid or ester should bepresent in the feed in from 0.1 to 20 mole percent. From the standpointof achieving a reasonable throughput combined with an acceptable yield,the concentration of the reactant in the feed is from about 3-30 molepercent. Concentration of reactant in the feed is controlled to a largedegree by the amount of inert gas present. The preferred inert gas ordiluent is nitrogen although other inert gases such as carbon dioxide,helium, argon, and the like are suitable. Air is a very convenientsource of oxygen plus inert diluent.

Another important parameter is contact time in the process of thisinvention. Contact or reaction time is defined for the purpose of thisinvention as the catalyst volume divided by the volume of gas feed persecond at the reaction temperature. The catalyst volume is the bulkvolume occupied by the catalyst in the reactor. The term catalyst inthis sense not only includes the material identified by the empiricalformula above but also includes the support material if present.Accordingly, reaction times can range from 0.05 to 3.0 seconds and moregenerally in the range of from 0.1 to 1.0 second. The reaction ispreferably carried out at or near atmospheric pressure although the useof higher pressures up to about 10 atmospheres is contemplated.

The process of this invention is further illustrated in the followingspecific examples.

EXAMPLE I

This example illustrates the use of the slurry method for preparing thecatalysts of this invention. A slurry of 20.20 g. Fe(NO₃)₃.9H₂ O, 4.45g. of Sb₂ O₅, and 6.38 g. of WO₃ in 100 ml. of water was dried overnightat 120° C. The nitrates in the resulting solid were decomposed at 350°C. for 24 hours and the resulting solid was ground under acetone andcalcined at 980° C. for two days. The use of this and similarly preparedcatalysts in the oxydehydrogenation reaction is described in thefollowing examples.

EXAMPLE II

This example illustrates the use of the catalysts described in Example Iin the oxydehydrogenation of isobutyric acid to produce methacrylicacid. The procedure consisted of feeding a preheated mixture ofisobutyric acid, air and acetone. 0.5 g. of catalyst was run in areactor which was a stainless steel tube 2" long with 1/2" O.D. and 3/8"I.D. The feed mixture composed of 1080 cc. of water, 336 cc. ofisobutyric acid, and 55 cc. of acetone was fed at a rate of 5.7 cc. perhour. The oxygen (as air) was fed at the same time at the rate of 20standard cc. per minute. The reaction temperatures used and resultsobtained with various catalysts are given in the following table.

                  TABLE                                                           ______________________________________                                                     Reaction % Conversion                                                                              % Selectivity                                            Temp.,   of Isobutyric                                                                             to Methacryl-                               Catalyst     °C.                                                                             Acid        ic Acid                                     ______________________________________                                        FeSb.sub.0.5 W.sub.0.5 O.sub.x                                                             415      22.63       54.26                                       FeNb.sub.0.5 Ta.sub.0.5 O.sub.x                                                            418      32.94       52.76                                       FeSb.sub.0.33 Nb.sub.0.33 W.sub.0.33 O.sub.x                                               416      21.26       58.41                                       FeNb.sub.0.33 Ta.sub.0.33 W.sub.0.33 O.sub.x                                               416      15.26       52.27                                       ______________________________________                                    

We claim:
 1. A process for the catalytic conversion of isobutyric acidor a lower alkyl ester thereof to the corresponding α,β-olefinicallyunsaturated derivative by oxydehydrogenation wherein a catalyst iscontacted with a gaseous feed stream containing said acid or ester andmolecular oxygen at a temperature between about 300° and 500° C. whichcomprises using as catalyst a material having the gramatom empiricalformula FeM_(a) O_(x) wherein a is a number from 0.5 to 3 and M is atleast two members selected from the group consisting of antimony,niobium, tantalum and tungsten and x represents a number determined bythe valence requirements of the other elements present.
 2. The processof claim 1 wherein isobutyric acid is converted to methacrylic acid. 3.The process of claim 2 wherein the catalyst has the gram-atom empiricalformula FeSb₀.5 W₀.5 O_(x).
 4. The process of claim 2 wherein thecatalyst has the gram-atom empirical formula FeNb₀.5 Ta₀.5 O_(x).
 5. Theprocess of claim 2 wherein the catalyst has the gram-atom empiricalformula FeSb₀.33 Nb₀.33 W₀.33 O_(x).
 6. The process of claim 2 whereinthe catalyst has the gram-atom empirical formula FeNb₀.33 Ta₀.33 W₀.33O_(x).